Its aim in particular is to propose a particular lamp implementation which, while comprising a mask intended to prevent diffusion of light upwards, makes it possible to obtain a light intensity sufficient for satisfactory viewing of certain elements placed in different areas situated above the cutoff line of the beam emitted by the lamp.
The field of the invention is, in general terms, that of motor vehicle lamps. In this field, different types of lamp are known, amongst which there are essentially:
sidelights, of low intensity and range;
low beam, or dipped beam, headlights, of stronger intensity and with a range on the road close to 70 meters, which are used mainly at night and whose light beam distribution is such that it makes it possible to not dazzle the driver of an oncoming vehicle;
long-range high beam headlights, and additional long-range type lights, whose field of vision on the road is close to 200 meters, and which must be switched off when another vehicle is oncoming in order to not dazzle its driver;
fog lights.
The lamp device according to the invention is more particularly intended to be used as a low beam headlight, but the special nature of its structure, and in particular of its projection lens, could be reproduced on other types of lamp in order to meet different requirements.
In the prior art, essentially two types of lamp each having a distinct structure are known for low beam headlights.
The first type of lamp consists essentially of a reflector associated with a light source. The reflector consists of a mirror comprising a set of serrations, or areas of various shapes, thus producing a complex surface whose shape, which has previously formed the subject of precise calculations, makes it possible to reflect light signals emitted by the light source in order to produce a light beam essentially oriented horizontally and downwards.
The second type of lamp is illustrated in FIG. 1. This Figure depicts a sectional side view of a known low beam headlight 100 of the prior art. Such a low beam headlight comprises essentially a reflector 101, a light source 102, radiating power in the form of emitted light signals 103, disposed in the vicinity of the apex of the reflector 101, and an output surface 104 for a light beam 106. The output surface 104 can be for example a plastic type closure glass; preferably it has no optical properties, that is to say it does not deviate, or deviates very little, the light rays passing through it.
Before reaching the output surface 104, the light signals 103 are caused to pass through, either directly, or after reflection from the reflector 101, a lens 105. This lens is most often of convex type and circular. It is referred to as a projection lens. It has an input face 110 and an output face 111. It images the light beam 106, the orientation and range of which depend in particular on the disposition of the lens 105 within the lamp device 100 and the optical characteristics of the lens 105. Preferably, a central part of the light source 102 is disposed in the focal area of a first focus F1 of the reflector 101, and the focus of the projection lens 105 is situated in the focal area of a second focus F2 of the reflector 101. Thus, a light signal 103 emitted by the central part of the light source 102 will pass through the second focus F2 of the reflector 101 and will come out of the projection lens 105 horizontally or approximately horizontally. With the exception of the light signals that are reflected from ends 107 of the reflector 101, all the light signals 103 emitted by the central part of the light source 102 converge towards the second focus F2.
In general terms, the expression “light signals” designates all the light rays emitted by the light source 102, and “light beam” designates all the light rays that are actually emitted by a lamp at the output surface 104, or at the projection lens 105.
In this second type of lamp, a mask 108 is interposed between the reflector 101 and the projection lens 105. The mask 108 is disposed in a plane parallel to the projection lens 105, approximately at the object focal plane of the lens, so that the image of the mask is emitted at infinity. By virtue of the presence of such a mask 108, the light beam 106 which is actually emitted by the lamp device 100 is not emitted above a cutoff line determined by the shape of an upper part 109 of the mask 108.
FIG. 2 gives an example of the shape 200 of the light beam 106 projected on a screen. A cutoff line 201 marks the boundary between a low area where the light intensity is sufficient to illuminate the road and complies with the various regulations laid down, and a high area where the light intensity is almost zero. The cutoff line 201 has a change in height 203 in the region of a central axis 202 of the beam. The shape 201 depicted, with a light beam higher on the right-hand part of the projection, corresponds to that of a lamp for a vehicle travelling in a country where driving on the right is prescribed. In a country where driving on the left is prescribed, a shape would be obtained which, with respect to a vertical axis 202, would be symmetrical to that depicted.
The two types of lamp described are available on the market today. Motor vehicle manufacturers choose one or other of these types of lamp essentially according to aesthetic criteria, the two types of lamp not having the same appearance.
However, a problem arises with the second type of lamp described. This is because, although it is true that the light intensity must be low above the cutoff line 201, the various regulations nevertheless lay down that a minimum light intensity be emitted in certain directions situated above the cutoff line 201. In particular, various regulations lay down a minimum light intensity at certain points situated above the cutoff line, these points being referred to as gantry points, since they correspond approximately to places in the vicinity of which signs of motorway sign type are located when these signs are at a given visibility distance from the vehicle. For example, in one American regulation, there are three gantry points which are respectively situated at 2u4l, 4u8and 4u8r with respect to the optical axis of the lens and a line 1.5u1r to 3R, the figures corresponding to degrees, “u” corresponding to “up”, “l” corresponding to “left”, and “r” corresponding to “right”.
Various solutions have been proposed in the prior art to make it possible to illuminate these gantry points while retaining the mask 108 in the lamp device.
A first solution consists of providing a hole in the mask 108. If this hole is disposed in the correct place, an approximately rectangular illuminated area is then obtained above the cutoff line, this area containing the gantry points. The statutory requirements are thus satisfied, but the diffused light intensity in the rectangle is such that it is unpleasant—perhaps even a hindrance—for the driver.
A second known solution consist of slightly roughening the input face of the lens 105. Some of the light signals are thus deviated from their initial path and some are emitted in the direction of the gantry points. But such a procedure has a number of drawbacks: on the one hand, the roughened surface diffuses light almost isotropically, a large amount of energy being wasted, including in areas of the beam where the intensity is already relatively low; on the other hand, it is necessary to carry out a post-treatment of the lens after moulding. In practice, a surfacing operation has therefore to be performed in order to obtain a slightly roughened face, this operation following the moulding operation.
The device according to the invention answers the problems that have just been described. In general terms, the device according to the invention proposes a solution that makes it possible to provide in a controlled manner a light intensity at the gantry points and in the vicinity of these points, while retaining the presence of a mask in order to not dazzle oncoming motorists and retaining good homogeneity of the light beam produced by the lamp device for illuminating the road.
To that end, in the invention, a modification is proposed of the output surface of the projection lens, and more particularly of certain areas of this output surface.